1. Field of the Invention
The present invention relates to a bipolar transistor and a fabrication method thereof, and more particularly, to a bipolar transistor having beneath an emitter region an impurity region with the same conductivity type as that of the emitter region and a fabrication method thereof.
2. Description of Releated Art
FIG. 8 shows a sectional view of a prior art bipolar transistor.
On the surface of a P-type silicon substrate 1 there is selectively formed a silicon oxide film for element isolation, and the substrate 1 is subdivided into a plurality of element regions by the silicon oxide film 4. In each of the element regions there is provided a buried collector region 2 doped with an N-type impurity, and an epitaxial growth layer 3 with thickness of about 1 .mu.m is formed on the buried collector region 2. In the epitaxial growth layer 3 there is selectively formed a collector connecting region 2a which reaches from the surface of the layer 3 to the buried collector region 2.
On the epitaxial growth layer 3 there is selectively formed a base region 11, and an emitter region 15 is selectively formed on the surface of the base region 11. In addition, an insulating film 5 is formed on the epitaxial growth layer 3 exclusive of the formation region of the base region and on the silicon oxide film 4. On the insulating film 5 there is selectively formed a P-type polycrystalline silicon film 6 for base lead-out. The polycrystalline silicon layer 6 is electrically connected to the base region 11 via a P-type polycrystalline silicon film 10 which is selectively formed on the base region 11.
On the insulating film 5 inclusive of the part over the polycrystalline silicon film 6 there is formed a silicon oxide film 7. An aluminum electrode 14 fill in a contact hole 7a provided in the silicon oxide film 7, and is electrically connected to the polycrystalline silicon film 6. Further, over the base region 11 there is formed an opening 16 which reaches from the surface of the silicon oxide film 7 to the base region 11, and a silicon oxide film 12 is formed on the sidewalls of the opening 16. Moreover, an N-type polycrystalline silicon film 13 which is filled in the opening 16 is selectively connected to the emitter region 15. The N-type polycrystalline silicon film 13 is formed in such a manner as to extend somewhat over the silicon oxide film 7 from the interior of the opening 16. On the N-type polycrystalline silicon film 13 there is formed an aluminum electrode 14. Further, over the collector connecting region 2a there is provided a contact hole 7b which reaches from the surface of the silicon oxide film 7 to the collector connecting region 2a, and the contact hole 7b is filled in with the aluminum electrode 14 which is formed on the silicon oxide film 7 according to a predetermined pattern.
In order to operate at high speed the elements of the prior art bipolar transistor constructed as in the above, it becomes necessary to suppress the reduction in the cut-off frequency due to the base push-out effect (the so-called Kirk effect) at high current domain. For this purpose, one may try to increase the concentration of the N-type impurity in the epitaxial growth layer 3, or dope N-type impurity atoms to the portion of the epitaxially deposited layer 3 which is below the emitter region 15. However, if the concentration of the N-type impurity for the entirety of the epitaxial growth layer 3 is increased, the epitaxial growth layer with high impurity concentration is brought into contact with a wide range of the bottom surface of the base region 11, so that, although the cut-off frequency can surely be made high, the capacity between the collector and the base is increased, which obstructs the increase in the operating speed of the elements. Further, the breakdown voltage between the collector and the base is also deteriorated. Therefore, this approach cannot be said realistic.
Now, the epitaxial growth layer 3 is formed by epitaxially growing silicon on the buried collector region 2 at a temperature above 900.degree. C. Since impurity atoms diffuse by auto-doping to the epitaxial growth layer 3 from the buried collector region 2 during the time of formation of the epitaxial growth layer, the impurity concentration in the epitaxial growth layer 3 becomes high also in the region other than the region where the N-type impurity atoms are added. As a result, the collector-base capacity between the base region 11 and the region of the epitaxial growth layer 3 exclusive of the region of addition of the N-type impurity atoms becomes high, making a full increase of the operating speed of the elements unachievable.